STERILISATION 



89 



diagram that the water is in communication at a, b, c, d, and e, hence 

 the water at a, b, and c will always be at the same level, and the supply 

 of water for the steam is drawn from the water in the pan A. The 

 steam issuing from C is carried up between the two walls of the 

 apparatus (/) and at the top enters the chamber through small 

 apertures in the inner wall (g). The chamber thus becomes filled with 

 steam. As shown in the diagram, the bottom of the chamber also has 

 two walls, and the inner of the two communicates with the pan A by 

 means of the tube (h), so that all the steam condensed in the chamber 

 drops back into the pan A in the form of water, instead of escaping 

 into the room in the form of vapour. 



In both forms of sterilisers the objects to be sterilised are usually 

 placed in wire cages, either round or rectangular so that they can be 

 easily lifted in and out. By this method 

 of moist heat all nutrient media, all 

 liquids, and all substances which cannot 

 stand a higher temperature than 100° C. 

 without being spoilt, are sterilised. This 

 is the most convenient method of freeing 

 dusters, articles of clothing, etc., from 

 germs. 



Sterilisation by moist is more effica- 

 cious than by dry heat, but a higher 

 temperature is possible by the latter 

 method, and is, therefore, preferable for 

 substances that can stand much heat 

 without injury. Again, in the case of 

 cotton wool, sterilisation must be effected 

 by dry heat, for moist cotton wool used 

 as a stopper for a flask containing a 

 nutrient medium is ineff"ective. Hence, 

 all glass ware and cotton wool are 

 sterilised by being placed in the dry- 

 air oven, which must be heated for at least 20-30 minutes at 120° C. 



We may use the naked flame of a Bunsen burner to sterilise knives, 

 platinum loops, platinum wires, glass rods, and other substances not 

 injured by coming into contact with the naked flame. 



Finally, a very efficient heat method is by the use of the Autoclave. 

 The principle of this apparatus depends on the fact that the boiling 

 point of water depends on the pressure. In the ordinary steam, 

 steriliser the water is heated at the atmospheric pressure at which 



Fig. 67. — Autoclave. 



